Treatment of wells



2,885,004 TREATMENT OF WELLS Robert 0. Perry, Tulsa, Okla., asslgnor to Sinclair )il & Gas Company, Tulsa, Okla., a corporation of Maine No Drawing. Application November 2, 1955 Serial No. 544,596

3 Claims. (Cl. 166-42) This invention relates to a three stage method of acidizing wells drilled in sandstone formations. In a particular aspect, this invention relates to a three stage method of acidizing water intake wells drilled in sandstone formations whereby high permeability is effected in areas of low permeability.

It is common knowledge that one of the major problems in the production of oil is increasing recovery of oil after primary flow and pumping methods are exhausted. Increased oil recovery is frequently attempted by secondary methods carried out by pumping water under pressure into an input well to .drive additional oil into adjacent output wells for recovery. One of the disadvantages of this method is that there is almost always some type of filter cake or tight sands which lower the permeability of the formation and thus interfere with the movement of the water drive. Consequently, in an effort to overcome this problem, various methods of acidization have been proposed for increasing water intake rates in secondary recovery operations, particularly where the producing formation consists largely of sandstone or other siliceous rock. Generally, these procedures consist in introducing into the geological formation a fluid reagent such as hydrochloric acid or hydrofluoric acid, either alone or in combination, for the purpose of causing the acid to dissolve the siliceous structure of the well and thereby open up the well formation. In U.S. Patent No. 2,367,350, there is disclosed a method of increasing the permeability of a geological formation containing both silicates and carbonates by first treating the formation with hydrochloric acid and then injecting a mixture of hydrochloric acid and hydrofluoric acid, the theory being that the initial hydrochloric acid pretreatment eliminates the formation of precipitates which otherwise would be produced by the reaction of hydrofluoric acid on carbonates. Similarly, U.S. Patent No. 2,225,695 discloses a method of opening sandstone formations by treating the well first with hydrofluoric acid to attack the siliceous material and then treating-with hydrochloric acid to dissolve the silicic acid Patented May 5, 1959 '2 vention, I have found that a permanent change in the permeability of the well is effected and, that increased water intake rates may be employed in secondary recovery methods to drive additional oil from the retaining sands and other gelatinous reaction products resulting from the hydrofluoric acid treatment. While these methods and various other modifications have shown favorable results in some instances, in many others they have not effectively increased the permeability of siliceous formations sothat a suflicient amount of water may be injected injected into the desired strata.

I have discovered an improved method for the treatment of wells penetrating sandstone formations, particularly water input wells on secondary recovery projects,

by means of a three-stage method which consists essentially of contacting the strata with hydrochloric acid of defined concentration prior to and following the treatment-of the formation with a mixed acid solution consisting essentially of hydrochloric acid and hydrofluoric acid of specified strengths. By the method of my inthrough output wells. In my method it is essential that each of the three acid treating steps be employed as they are interdependent in increasing the permeability of certain strata where other methods of acidizing are not effective. My method is particularly useful for treating a plugged sandstone strata in conjunction with a water drive operation although it is applicable to treating input wells beforethe water drive is begun and to treating any other sandstone formation.

The process of my invention will be illustrated by reference to the following three-stage acidizing procedure carried out in a sandstone formation where primary methods of recovery had been abandoned and secondary methods including water flooding had been practiced to decrease the permeability of the input well strata.

A cased well drilled to sandstone at'a depth of 730 feet was provided with 715 feet of a inch Saran acid injection line inside and projecting from the well tubing and a packer was set at 680feet around and near the end of the well tubing. The Saran line weighted 2.5 pounds per hundred feet and the bottom 27 feet of the line were perforated to facilitate distribution of acid. A four foot, one-half inch lead filled pipe weighing 12 pounds was spliced to the bottom end of the line in order to counteract vertical forces against its entry.

For treating the well in accordance with the invention, 52 gallons of concentrated muriatic acid were pumped into the well over a period of four hours. Water of dilution was passed down the well tubing to provide a final acid concentration of 5 percent by volume contacting and passing into the strata. The original intake rate prior to acid injection was 43 barrels per day at 390 p.s.i. and

after the acid treatment the rate dropped to 36 barrels per day at a pressure of 335 p.s.i. The acid was allowed to remain in position until its reaction with the sandstone formation was complete and a second acid charge was then injected.

The second charge consisted of 104 gallons of hydrochloric acid and 35.5 gallons of hydrofluoric acid, both I acids having a final acid concentration of 5 percent by volume when mixed with separately passed water of dilution. A maximum intake rate of 67.4 barrels per day at 400 p.s.i. was obtained after ninehours of treatment. The input rate started to decline and after 11 hours the hydrofluoric acid injection was stopped. The rate continued to decline and the hydrochloric acid injection was stopped after fourteen and one-half hours, the input rate having declined to as low as 51 barrels per day at 400 p.s.i. Ten hours later the well was opened and the Saran tube removed. The well was allowed to backflow for about.30 minutes and then shut-in. Four hours later the input rate increased to 67 barrels per day and thereafter leveled to a rate of 66.6 barrels per day five days later.. The well was then acidized with a third treatment of acid consisting of a 5 percent solution of hydrochloric acid. Prior to final .acidization the intake rate of the well was 62.5 barrels per day at a wellhead pressure of 420 p.s.i. 33.6 gallons of hydrochloric acid were then injected into the formation and the rate increased to 74.5 barrels per day at a pressure of 428 p.s.i. A maximum input rate of 94.5 barrels per day at 460 p.s.i. was obtained in four hours and treatment was stopped one hour later after having injected a total of gallons of acid, the input rate being 86.5 barrels per day at a pressure of 462 p.s.i. The well was allowed to stabilize and twenty 'four'hou rs-later a final rate of 93 barrels per day at 455; In this test there was little permanent increase with a p.s.i. was obtained, I I I I I I I I I I 3% hydrochloric acid treatment. I I I I I I I I I I I In carryingcuttheprocessofmyinvention,a water I I II I solution containing :2 to percent by volume of hydro. Example H I I chloric acid may be used in the initial stage of'the opera- I f I I lion, a preferred concentration being about 3 to 6 percent;

' i p j 3 Higher acid I concentrations are avoided I since the :acid

. Two cores from the upper Spraberry formation were se-: I lected for this test. 1 The cores,'designated A and'B', were I I I I taken from; intervals of 7237.5 feet and 7185.1 feet, re-' 1 I I solution in contactwith cementltious materials,- :suchas. I Pe i dYI a th a ow u phys s c ra teristics; I I I clay, carbonates and silicates, causes them to swell and. I I I; I I I I I I I i I obstruct the capillaries in the formation; In; the: second 1.0 I I I I I I I stage treatment the mixed acid solution will contain about I I I I I I I I I I I I I I CoreA CoreB I 3 to 10 percent, preferably '5 to 8 percent, of hydrochloric v v I I I I M l acid and 3 to. 6 percent hydrofluoric, acid, a preferred I Length -.eentimeters; I I I concentration forthe hydrofluoric acid being aboutS per. 3 I ,333, cent; In the third stage treatment,- the acidlzation 1s carvolumecfcme I I f i I r I 7 I I I I I I f I 'ried out m substantially the same mannerpreviously described for the initial'hydrochloric acid treatment-Le. byI P S Y I injecting an aqueous solution of hydrochloric acid of 2 I i I I to 10 percent concentration, preferably about 3. to 6 pen:

I cent, against-the wel1;strata-. In applying the rocessiof I my invention, the'partially depleted formation mayhavepercenm 1 0 320 The cores were, put underI'flood, with 50,000 p.pim. brine I I I and Ia'ft'er 5 hours the :permeability of. Core 1A was 05155 I 5 f I? theca'singor welltubingiz I I II II I I will-be: further illustrated 1 been: subjected to conventional water drive in. which in- I stance :the existing pattern of inputzwells. may :be iern-j I I I I I I I I I pl oyed. Usually, a5 spot or '9 spot pattern: is adopted-in I 1 I i f 1 I I r I i I 3 i order-to promote uniformity. of flow and reduce drive 1 v distances. Theinnut well is treated: underregular input I I pressure and the dilute acidscan be pumped-directly into I the acid injection pipeIat the well head. During each:

I stage of thetreatment, the-acidization is carried: out until I I the input rate has. leveled out. or until the desired rate is;

I obtained. lnorderthat the hydrochloric acid orthe mire II I I I 'ture of; hydrochloric acid and hydroliuo ricacidpenetratei I I I I I I I I I I 1 only sand formations containing oil and gas, the; acid isv I I I I 3 i l introduced throughza. small plastic tube madeof Saran or polystyrene which iszlowered within the regular-inputpipe to the bottornof the Well :or the desired area: Pan I I I I I I I I I I T ticnlarly in this method, water of dilution is passed down" sandstone formations in accordance with the present in vention and Examples I to IV, inclusive, represent typical acidizing procedures.

Example I j A core sample from the interval of 525.0 feet to 525.4 feet taken from an input well had the following physical characteristics:

I millidarcy and 0.1025 for Core '3; The flood was i I I changed to 2% hydrochloric acid and 3% hydrofluoric acid in 50,000 p.p.m: of brine I and the permeability I I I checked every ten minutes. The flow declined rapidly and after-80. minutes had droppedto'.0036 millidarcy on I I :core A Band 10038 millidarcy on: Core:B. The test was I I I I shut down over night and next morning :anattempt :was' 'rnadeitoestahlisha flow with 50,000 p.p.m. brine; There I I :was n'o' flo'wi- The: total through-put was 20.5 cc. for I I I I: C ore A, and 13 cc. for: Cor-e13; The :test was stopped and the: 'cores examined; 1 Except :for, a very: shallow penetration of about 1. min: of bleaching 'there was' nc: I I I I I change in the physicalappearance of the; samples; I 1 f r i I core sampleif el tq- I The process of my invention physilcal: eharacteristicsz: I I I I I I I I I I I I I I I by reference to the following data developed onactuai 40 I I I I I I I I I v I I I I P I I I sand'cores' obtain-ed fromsandstone formations on sec- Length I --I cm I7 ()5. ondary recovery projects. In each of the following ex- Diameter cm 5,06 amples a cylindrical core sample was taken and perme- Diameter of hole cm 1,26 ability determined by conventional apparatus consisting Volume of core cc 132,8 of means for holding the core and means for forcing the Pore volume cc 15.5 fluid therethrough at various pressures and temperatures. Permeability mdcys 0,32 Examples V and VI illustrate the method of acidizing Porosity 11,7

The core was put under flood with 50,000 p.p.m. brine until permeability was steady at 0.177 millidarcy. The feed was changed to 3% hydrofluoric acid in 50,000 p.p.m. brine and the permeability checked every ten minutes. After 90 minutes the flow had declined to .0037 millidarcy. The test was stopped and the core examined. There was no change in the physical appearance of the Length CII1 11.4 sample O.D I-- Example IV I.D. cm 2.54

Core volume cc 1220 A core sample from the interval of 524.6 to 525.0

Pore volume (with 20% porosity) cc 244 feet from the input well of Example I had the following physical characteristics:

The core was flooded with tap water to obtain a base line ermeability and then with a 3% solution of hydrochloric Length cm 11.5 acid, After flooding through 5.75 pore volumes of 3% OD. cm 12.0 hydrochloric acid, the feed was changed to tap Water. IID. cm 1.3 The following data show the results of the test when Core Volume cc 1284 plotting pore volume against permeability. Pore volume (with 20% porosity) cc 257 In this test the core was flooded with tap water and then Feed Pore gggfi; with flood B injection water to obtain a base line per- Volume (millimeability. The core was then put under flood with 3% dmces) hydrochloric acid, a mixed acid solution of 3% hydro- Ta Water 11 5 88 chloric acid and 3% hydrofluoric acid, and last with 3 H drocii1ii'ri'i'dIIIIII:IIIIIIIIIIIIIIII 5375 i190 A final flood Tap Water 6A 0 J6Ctl0n Water was made to determine the increase in p'erme ability:

9 taken fl'flnl the; interv'alof- 7180.5 feet I I I I I I I in the upper Spraberry: formation :had, the. following plotted against pore volume when acidizing in accordance with the above procedure.

Permea- Feed Pore bility Volumes (millidarcles) Tap Water 27. 2 6 Flood "B Water 6.0 67 3% Muriatic Acid in Flood "3 Water 7. 2 2. 70 3% Muriatic Acid and 3% HF in Flood "B" Water 7. 2 10. 42 2% Muriatic Acid in Flood "3 Water.... 5. 6 12. 50 ap Water 6. 8 10. 50

In the final flood with tap water there was a permanent increase in permeability of 17.1 times greater than the original base line permeability of .6.

Example V A sandstone core selected from an interval of 632 feet had the following physical characteristics:

Weight grams 1073.25 Diameter cm 9.50 Diameter of hole cm 1.30 Length cm 7.45

Permea- Feed Time bllity (Hours) (millidarcies) 3.077 hydrochloric acid in brine l 72 4. 42 3.01% hydrochloric acid and a 05% hydrofluoric acid in brine l 30 28. 2 8% hydrochloric acid in brine l 15 75. 6

l The brine contained 50,000 p.p.m. oi NaOl.

Inspection of the core revealed that it had retained its shape and was soft enough to be penetrated with a knife.

I claim:

1. A three-stage method of acidizing wells drilled in sandstone formations to increase the permeability there- 01, comprising the steps of contacting said formation with a first solution of hydrochloric acid of about 2 to 10 percent strength and allowing the acid to react with the sandstone formation; contacting said formation with a second acid solution of hydrochloric acid of about 3 to 10 percent strength and hydrofluoric acid of about 3 to 6 percent strength, and allowing the second acid solution to react with the sandstone formation until the permeability of the formation is increased; and thereafter contacting said formation with a third acid solution of hydrochloric acid of about 2 to 10 percent strength and allowing the third acid solution to react with the sandstone formation.

2. The method in accordance with claim 1 wherein the sandstone well is an input well in which the permeability has been decreased by the effect of water injection.

3. The method of claim 2 wherein said first and third solutions are hydrochloric acid of about 3 to 6 percent strength and said second solution contains hydrochloric acid of about 5 to 8 percent strentgh and hydrofluoric acid of about 3 to 6 percent.

References Cited in the file of this patent UNITED STATES PATENTS 

